Can Giant Planets Form by Direct Gravitational Instability

Gravitational instability has been invoked as a possible mechanism of the giant planet production in protoplanetary disks. Here we critically revise its viability by noting that to form planets directly, it is not enough for protoplanetary disks to be gravitationally unstable. They must also be able to cool efficiently (on a timescale comparable to the local disk orbital period) to allow the formation of the bound clumps by fragmentation. A combination of the dynamical and thermal constraints puts very stringent lower limits on the properties of disks capable of fragmenting into the self-gravitating objects: for the gravitational instability to form giant planets at 10 AU in the disk cooled by the radiation transfer, the gas temperature must exceed 103 K with a minimum disk mass of 0.7 M☉ and a luminosity of 40 L☉. Although these requirements are relaxed in the more distant parts of the disk, masses of the bound objects formed as a result of instability are too large even at 100 AU (~10MJ) to explain the characteristics of known extrasolar giant planets. Such protoplanetary disks (and planets formed in them) have very unusual observational properties, and this severely constrains the possibility of giant planet formation by direct gravitational instability.

[1]  C. Hayashi.  Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula , 1981 .

[2]  A. Cameron,et al.  Hydrodynamic instability of the solar nebula in the presence of a planetary core , 1974 .

[3]  H. Mizuno,et al.  Formation of the Giant Planets , 1980 .

[4]  A. G. W. Cameron,et al.  Physics of the primitive solar accretion disk , 1978 .

[5]  A. Boss.  Convective Cooling of Protoplanetary Disks and Rapid Giant Planet Formation , 2004 .

[6]  Charles F. Gammie,et al.  Nonlinear Outcome of Gravitational Instability in Cooling, Gaseous Disks , 2001, astro-ph/0101501.

[7]  T. Quinn,et al.  Formation of Giant Planets by Fragmentation of Protoplanetary Disks , 2002, Science.

[8]  D. Lin,et al.  USING FU ORIONIS OUTBURSTS TO CONSTRAIN SELF-REGULATED PROTOSTELLAR DISK MODELS , 1993, astro-ph/9312015.

[9]  Drake Deming,et al.  Scientific Frontiers in Research on Extrasolar Planets , 2003 .

[10]  P. Cassen,et al.  The Effects of Thermal Energetics on Three-dimensional Hydrodynamic Instabilities in Massive Protostellar Disks , 1998 .

[11]  S. Beckwith,et al.  A Survey for Circumstellar Disks around Young Stellar Objects , 1990 .

[12]  Fast Accretion of Small Planetesimals by Protoplanetary Cores , 2003, astro-ph/0311440.

[13]  R. Sari,et al.  Final Stages of Planet Formation , 2004, astro-ph/0404240.

[14]  A. Toomre,et al.  On the gravitational stability of a disk of stars , 1964 .

[15]  P. J. Armitage,et al.  The effect of cooling on the global stability of self-gravitating protoplanetary discs , 2003 .

[16]  Alan P. Boss,et al.  Evolution of the Solar Nebula. IV. Giant Gaseous Protoplanet Formation , 1998 .

[17]  M. Tamura,et al.  Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a 1 Arcsecond Imaging Survey: Evolution and Diversity of the Disks in Their Accretion Stage , 2002 .